The present invention relates to methods and apparatus for use in an electron microscope to improve the detection of electron images, and more particularly to methods and apparatus for converting and using the energy of the electrons in the image to improve image resolution and reduce noise.
Electron microscopes use a beam of accelerated electrons which pass through or are deflected by a sample to provide an electron image and/or diffraction pattern of the sample. To provide a record of these images and/or diffraction patterns, at least a portion of the kinetic energy of such electrons is converted into some other form of energy which can be measured and permanently stored. One example of such an energy conversion process is the excitation of silver halide grains in a photographic emulsion. Chemical development converts the grains into a permanent distribution of silver particles, the density of which can be measured by commercially-available microdensitometers.
Another example of an energy conversion process for the electrons is the generation of light images by impinging the electrons onto scintillator materials (e.g., phosphors), and then capturing the light images and/or patterns onto a two-dimensional imaging sensor. One example of such an imaging sensor is a charge coupled device (CCD). The output from the CCD may be read as an analog signal, measured by an analog to digital converter, and then displayed (such as on a video monitor) and/or stored permanently (such as in the memory of a computer).
These two examples employ different means to convert and store the relative intensities of the electrons. However, the process of the deposition of the electrons' energies is the same. That is, once an accelerated electron enters the solid volume of the detector (photographic film emulsion or scintillator film), it starts to lose energy to the solid. This energy loss is at a rate which depends on the initial energy of the electron and the solid material through which it is traveling. The electron is also scattered randomly by the fields surrounding the atoms of the detector in a manner which alters the electron's direction or path of travel.
The result is that a series of accelerated electrons of the same initial energy, entering the solid detector at a specific point, will generate a set of paths which together fill a region of space resembling a cloud. This cloud-shaped volume can be defined as the envelope of all possible paths and is termed the interaction volume of the electron beam in the detector. The energy of the electron beam and the average atomic number density (Z density) of the detector material together determine the electron path's average behavior and thus the size and shape of the interaction volume.
Higher electron energies cause the interaction volume to be larger, while denser materials in the detector will cause it to be smaller. Denser materials also increase the average deflection angles of electrons and therefore cause more scattering of electrons back out of the detector. The calculation of paths the electrons will take and their resulting statistics is known in the art as "Monte Carlo" simulation.
The interaction of high energy electrons with the volume of the solid material of the detector generates spreading and noise which constitute primary limitations on the amount of spatial and intensity information obtainable from the incident electron image. One approach to dealing with the non-zero interaction volume of the detector has been to make the detector as thin as possible. In a thin sheet of film, for example, a beam of accelerated electrons experiences minimal scattering before exiting.
However, a disadvantage of using a thin film of the detector material is that only a small fraction of each electron's energy is utilized. Making the detector thicker increases sensitivity, but also increases scattering and degrades resolution. Further, where a scintillator is optically coupled to a CCD (such as, for example, by a fiber optic), the scintillator can be made thin so that light is generated only in a small volume near the point of entrance of the electrons. However, the electrons continue to be scattered after leaving the scintillator, with some electrons being back-scattered into the scintillator again.
Such back-scattered electrons will cause scintillation as well, creating an extended, noisy flare around the central spot of light created by the electrons in their incoming traversal of the scintillator. One solution to the problem of back-scattering is taught by Mooney et al, U.S. Pat. No. 5,635,720. There, the fiber optic which couples the scintillator to the detector is replaced by a light metal fold mirror and a lens coupling. Thus, the number of back-scattered electrons reentering the scintillator is reduced, but at the cost of a decrease in light gathering efficiency compared to fiber optics which decreases the overall sensitivity of the camera.
Thus, the need remains in this art for a method and apparatus for reducing the contribution to the total noise and resolution loss caused by the initial step of electron interaction with the detector without sacrificing sensitivity in light collection.